1. Field of the Invention
This invention relates to bypass cryogenic leads. More particularly, it relates to such power leads incorporation high temperature, ceramic superconductors.
2. Background Information
A bypass current lead as employed for example in the spools of a Superconducting Super Collider (SSC), provides a paralleled current pass for current to bypass the quenched magnets and to follow through the unquenched string of magnets. This allows the energy in the quenched and the unquenched magnets to be dissipated with a different time constant-faster dissipation in the quenched magnets and slower dissipation in the unquenched ones-and therefore enhances the reliability and prolongs the useful life of the magnet. The bypass lead carries current only during a short period, following a magnet quench till the energy in the magnets is completely dissipated. In the SSC, the current pulse to the bypass leads rises to about 7 kA in several seconds and then decays to zero with an L/R time constant of about 30 seconds. The bypass leads are required to have a minimal heat leak under normal conditions when it carries no current, be ready at all times to permit very high magnitude of current flow in the event of a quench, and be ready for operation again within a short time (30 minutes in the case of SSC) following completion of a quench protection cycle. Separate flow rate control valves, small flow rate for normal operating conditions and large flow rate following a magnet quench, are available for controlling the cooling helium flow rate through the bypass lead. FIG. 1 shows the bypass lead and the cold mass of the spool pieces used in the collider ring of the SSC known as SPXA/SPRA spools.
In order to minimize the heat leak, a conventional bypass current lead, as illustrated in FIG. 2, employs stainless steel in its entire operating length, and uses copper only in the upper and the lower terminal regions, as shown schematically in FIG. 3. In carrying current following a magnet quench, Joule heating in the stainless steel part of the lead will increase significantly the local lead conductor temperatures. In order to prevent this temperature rise from causing a quench in the superconducting power bus which carries current to the unquenched string of magnets, a quench stopper is installed between the bypass lead lower terminal and the superconducting power bus. The quench stopper (13), as shown in FIG. 1, consists essentially of a big thermal mass, has complicated design feature for heat transfer consideration and is expensive to fabricate. In addition, low heat leak condition also requires that the bypass lead conductor length be sufficiently long and therefore needs to be wrapped a few times around the cold mass of the spools. This creates difficulties in lead design and spool component assembly.
There remains a need to eliminate the need of the quench stopper, to shorten the length of the bypass lead such that no wrapping around the cold mass is necessary and to reduce heat leak in normal conditions by employing ceramic high temperature superconductor in the bypass lead.